the clouds, reflects none. (See fig.) The temporary removal of both the strata, but more of the upper than the lower, he supposes effected by powerful upward currents of the atmosphere, arising, perhaps, from spiracles in the body, or from local agitations.

(390.) When the spots are attentively watched, their situation on the disc of the sun is observed to change. They advance regularly towards its western limb or border, where they disappear, and are replaced by others which enter at the eastern limb, and which, pursuing their respective courses, in their turn disappear at the western. The apparent rapidity of this movement is not uniform, as it would be were the spots dark bodies passing, by an independent motion of their own, between the earth and the sun; but is swiftest in the middle of their paths across the disc, and very slow at its borders. This is precisely what would be the case supposing them to appertain to and make part of the visible surface of the sun's globe, and to be carried round by a uniform rotation of that globe on its axis, so that each spot should describe a circle parallel to the sun's equator, rendered elliptic by the effect of perspective. Their apparent paths also across the disc conform to this view of their nature, being, generally speaking, ellipses, much elongated, concentric with the sun's disc, each having one of its chords for its longer axis, and all these axes parallel to each other. At two periods of the year only do the spots appear to describe straight lines, viz. on and near to the 11th of June and the 12th of December, on which days, therefore, the plane of the circle, which a spot situated on the sun's equator describes (and con sequently, the plane of that equator itself,) passes through the earth. Hence it is obvious, that the plane of the sun's equator is inclined to that of the ecliptic, and intersects it in a line which passes through the place of the earth on these days. The situation of this line, or the line of the

nodes of the sun's equator as it is called, is, therefore, defined by the longitudes of the earth as seen from the sun at those epochs, which are respectively 80° 21′ and 260° 21′ (=80° 21' +180°) being, of course, diametrically opposite in direction.

(391.) The inclination of the sun's axis (that of the plane of its equator) to the ecliptic is determined by ascertaining the proportion of the longer and the shorter diameter of the apparent ellipse, described by any remarkable, well-defined spot; in order to do which, its apparent place on the sun's disc must be very precisely ascertained by micrometric measures, repeated from day to day as long as it continues visible (usually about 12 or 13 days, according to the magnitude of the spots, which always vanish by the effect of foreshortening before they attain the actual border of the disc-but the larger spots being traceable closer to the limb than the smaller.') The reduction of such observations, or the conclusion from them of the element in question, is complicated with the effect of the earth's motion in the interval of the observations, and with its situation in the ecliptic, with respect to the line of nodes. For simplicity, we will suppose the earth situated as it is on the 10th of March, in a line at right angles to that of the nodes, i. e. in the heliocentric longitude 170° 21', and to remain there stationary during the whole passage of a spot across the disc. In this case the axis of rotation of the sun will be

situated in a plane passing through the earth and at right angles to the plane of the ecliptic. Suppose C to represent the sun's centre, P C p its axis, E C the line of sight, PN QApS a section of the sun passing

The great spot of December, 1719, is stated to have been seen as a notch in the limb of the sun.

through the earth, and Q a spot situated on its equator, and in that plane and consequently in the middle of its apparent path across the disc. If the axis of rotation were perpendicular to the ecliptic, as N S, this spot would be at A, and would be seen projected on C, the centre of the sun. It is actually at Q, projected upon D, at an apparent distance C D to the north of the centre, which is the apparent smaller semi-axis of the ellipse described by the spot, which being known by micrometric measurement, C D

the value of or the cosine of QC N, the inclination of the sun's CN equator becomes known, C N being the apparent semi-diameter of the sun at the time. At this epoch, moreover, the northern half of the circle described by the spot is visible (the southern passing behind the body of the sun,) and the south pole p of the sun is within the visible hemisphere. This is the case in the whole interval from December 11th to July 12th, during which, the visual ray falls upon the southern side of the sun's equator. The contrary happens in the other half year, from July 12th to December 11th, and this is what is understood when we say that the ascending node (denoted ) of the sun's equator lies in 80° 21' longitude-a spot on the equator passing that node being then in the act of ascending from the southern to the northern side of the plane of the ecliptic-such being the conventional language of astronomers in speaking of these matters.

(392.) If the observations are made at other seasons (which, however, are the less favourable for this purpose the more remote they are from the epochs here assigned); when, moreover, as in strictness is necessary, the motion of the earth in the interval of the measures is allowed for (as for a change of the point of sight); the calculations requisite to deduce the situation of the axis in space, and the duration of the revolution around it, become much more intricate, and it would be beyond the scope of this work to enter into them. According to the best determinations we possess, the inclination of the sun's equator to the ecliptic is about 7° 20' (its nodes being as above stated), and the period of rotation 25 days 7 hours 48 minutes.2

(393.) The region of the spots is confined, generally speaking, within about 25° on either side of the sun's equator; beyond 30° they are very

1 See the theory in Leland's Astronomy, art. 3258, and the formulæ of computation in a paper by Petersen Schumacher's Nachrichten, No. 419.

Bianchi (Schumacher's Nach. 483), agreeing with Laugier. Lelambre makes it 25 Oh 17; Petersen, 25d 4h 30m. The inclination of the axis is uncertain to half a degree, and the node to several degrees. The continual changes in the spots them selves cause this uncertainty.

rarely seen; in the polar regions, never. The actual equator of the sun is also less frequently visited by spots than the adjacent zones on either side, and a very material difference in their frequency and magnitude subsists in its northern and southern hemisphere, those on the northern preponderating in both respects. The zone comprised between the 11th and 15th degree to the northward of the equator is particularly fertile in large and durable spots. These circumstances, as well as the frequent occurrence of a more or less regular arrangement of the spots, when numerous, in the manner of belts parallel to the equator, point evidently to physical peculiarities in certain parts of the sun's body more favourable than in others to the production of the spots, on the one hand; and on the other, to a general influence of its rotation on its axis as a determining cause of their distribution and arrangement, and would appear indicative of a system of movements in the fluids which constitute its luminous surface bearing no remote analogy to our trade winds-from whatever cause arising. (See art. 239. et seq.)

(394.) The duration of individual spots is commonly not great; some are formed and disappear within the limit of a single transit across the disc-but such are for the most part small and insignificant. Frequently they make one or two revolutions, being recognized at their reappearance by their situation with respect to the equator, their configurations inter se, their size, or other peculiarities, as well as by the interval elapsing between their disappearance at one limb and reappearance on the other. In a few rare cases, however, they have been watched round many revolutions. The great spot of 1779 appeared during six months, and one and the same group was observed in 1840 by Schwabe to return eight times.' It has been surmised, with considerable apparent probability, that some spots, at least, are generated again and again, at distant intervals of time, over the same identical points of the sun's body (as hurricanes, for example, are known to affect given localities on the earth's surface, and to pursue definite tracks). The uncertainty which still prevails with respect to the exact duration of its rotation renders it very difficult to obtain convincing evidence of this; nor, indeed, can it be expected, until by bringing together into one connected view the recorded state of the sun's surface during a very long period of time, and comparing together remarkable spots which have appeared on the same parallel, some precise periodic time shall be found which shall exactly conciliate numerous and wellcharacterized appearances. The inquiry is one of singular interest, as there can be no reasonable doubt that the supply of light and heat

'Schum. Nach. No. 418, p. 150. The recent papers of Biela, Capocci, Schwabe, Pastorff, and Schmidt, in that collection, will be found highly interesting.

afforded to our globe stands in intimate connexion with those processes which are taking place on the solar surface, and to which the spots in some way or other owe their origin.

(395.) Above the luminous surface of the sun, and the region in which the spots reside, there are strong indications of the existence of a gaseous atmosphere having a somewhat imperfect transparency. When the whole disc of the sun is seen at once through a telescope magnifying moderately enough to allow it, and with a darkening glass such as to suffer it to be contemplated with perfect comfort, it is very evident that the borders of the disc are much less luminous than the centre. That this is no illusion is shown by projecting the sun's image undarkened and moderately magnified, so as to occupy a circle two or three inches in diameter, on a sheet of white paper, taking care to have it well in focus, when the same appearance will be observed. This can only arise from the circumferential rays having undergone the absorptive action of a much greater thickness of some imperfectly transparent envelope (due to greater obliquity of their passage through it) than the central. But a still more convincing and indeed decisive evidence is offered by the phænomena attending a total eclipse of the sun. Such eclipses (as will be shown hereafter) are produced by the interposition of the dark body of the moon between the earth and sun, the moon being large enough to cover and surpass, by a very small breadth, the whole disc of the sun. Now when this takes place, were there no vaporous atmosphere capable of reflecting any light about the sun, the sky ought to appear totally dark, since (as will hereafter abundantly appear) there is not the smallest reason for believing the moon to have any atmosphere capable of doing so. So far, however, is this from being the case, that a bright ring or corona of light is seen, fading gradually away, as represented in Pl. I. fig. 3., which (in cases where the moon is not centrally superposed on the sun) is observed to be concentric with the latter, not the former body. This corona was beautifully seen in the eclipse of July 7, 1842, and with this most remarkable addition witnessed by every spectator in Pavia, Milan, Vienna, and elsewhere: there distinct and very conspicuous rose-coloured protuberances (as represented in the figure cited) were seen to project beyond the dark limb of the moon, likened by some to flames, by others to mountains, but which their enormous magnitude (for to have been seen at all by the naked eye their height must have exceeded 40,000 miles), and their faint degree of illumination, clearly prove to have been cloudy masses of the most excessive tenuity, and which doubtless owed their support, and probably their existence, to such an atmosphere as we are now speaking of. (396.) That the temperature at the visible surface of the sun cannot